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The changing rotation period of comet 67P/Churyumov-Gerasimenko controlled by its activity

Comet 67P/Churyumov-Gerasimenko currently spins with a period of about 12.4 hours, but its spin period used to be about 12.8 hours in 2002. It has been suggested that the difference is due to a period decrease caused by the comet's activity during its last close approach to the Sun in 2009. However, rather than the expected decrease in the spin period, recent navigation data from the Rosetta spacecraft has shown that the period has increased by 80.5 s as of Apr 13, 2015, as the comet approaches perihelion. Keller et al. estimate the water sublimation rate on each facet of a 3D model of the comet to calculate the global torque acting on the comet and infer changes in rotation rate. They show that the model qualitatively explains the modest increase in period observed in the first half of 2015 and predict that the rotation period should then decrease rapidly by about 13 minutes after perihelion. This will be directly tested by measurements in the next months.